1. Field of the Invention
The present invention relates to a coordinate measuring machine.
2. Description of Related Art
Typically known coordinate measuring machine includes: a probe that is adapted to move within a predetermined range, the probe having a contact point to be in contact with an object to be measured; a movement mechanism that holds and moves the probe; and a controller that controls the movement mechanism. The coordinate measuring machine measures the object by moving the contact point along the surface of the object to be measured while forcing the probe against the object (see, for instance, Document 1: JP-A-2008-89578).
The surface profile measuring instrument (coordinate measuring machine) disclosed in the Document 1 moves the contact point along the surface of the object while the probe is forced against the object to acquire displacements of a drive mechanism (movement mechanism) and the probe and synthesize the displacements to detect the position (measurement value) of the contact point. Further, the surface profile measuring instrument measures a surface profile of the object and the like based on the detected measurement value.
Here, an error between the displacement of the probe acquired by the surface profile measuring instrument and actual displacement of the probe inevitably results in an error in the measurement value. Accordingly, in the typical arrangement, the displacement of the probe acquired by the surface profile measuring instrument is corrected by predetermined correction parameters as shown in the following formula (1) to reduce the measurement error.
                              (                                                                      X                  PA                                                                                                      Y                  PA                                                                                                      Z                  PA                                                              )                =                              (                                                                                CX                    m                                                                                        CX                                          m                      -                      1                                                                                        …                                                                      CX                    1                                                                                        CX                    0                                                                                                                    CY                    m                                                                                        CY                                          m                      -                      1                                                                                        …                                                                      CY                    1                                                                                        CY                    0                                                                                                                    CZ                    m                                                                                        CZ                                          m                      -                      1                                                                                        …                                                                      CZ                    1                                                                                        CZ                    0                                                                        )                    ⁢                      (                                                                                X                    PB                    n                                                                                                                    Y                    PB                    n                                                                                                ⋮                                                                                                  Z                    PB                                                                                                1                                                      )                                              (        1        )            
Formula (1) shows the correction parameters CXm to CX0, CYm to CY0 and CZm to CZ0 defined by coefficients of polynomial equations of the displacements of the probe XPB, YPB and ZPB that approximate the corrected probe displacements XPA, YPA and ZPA. Incidentally, the correction parameters CXm to CX0, CYm to CY0 and CZm to CZ0 can be obtained by measuring a reference sphere having a known diameter with predetermined measurement conditions (e.g. a push amount, measurement direction and measurement speed).
Here, the “push amount” refers to a displacement of the probe when the object is measured while the probe is forced against the object. The “measurement direction” refers to a rotation direction of each section of divided movement path of the probe that is approximated by an arc.
However, when the displacement of the probe is corrected by the predetermined correction parameters, the coordinate measuring machine may not properly correct the displacement of the probe under a measurement condition different from that when the correction parameters are obtained, thus resulting in increase in the measurement error. Specific explanation will be given below with reference to drawings.
FIGS. 8A to 8D show measurement results when only the measurement condition of the push amount is changed. FIGS. 9A to 9D show measurement results when only the measurement condition of the measurement direction is changed. Incidentally, FIGS. 8A to 8D and 9A to 9D show the measurement results obtained by measuring a side geometry of a cylindrical master gauge with different measurement conditions, where the correction parameters are defined by coefficients for approximating the corrected displacement of the probe with a third-degree polynomial equation of the displacement of the probe (i.e. when n=3 in the above-mentioned formula (1)).
Further, FIGS. 8A to 8D respectively show the measurement results where, while representing the push amount in relative two stages (i.e. small and large), (i) the push amount for measurement (during a profiling measurement) and (ii) the push amount in acquiring the correction parameter (during probe calibration) are: (i) small and (ii) small in FIG. 8A; (i) small and (ii) large in FIG. 813; (i) large and (ii) small in FIG. 8C; and (i) large and (ii) large in FIG. 8D. FIGS. 9A to 9D show the measurement results where (i) the measurement direction during the profiling measurement and (ii) the measurement direction during the probe calibration are: (i) counterclockwise and (ii) counterclockwise in FIG. 9A; (i) counterclockwise and (ii) clockwise in FIG. 9B; (i) clockwise and (ii) counterclockwise in FIG. 9C; and (i) clockwise and (ii) clockwise in FIG. 9D.
When the measurement conditions during the probe calibration and the profiling measurement are the same, as shown in FIGS. 8A and 8D and FIGS. 9A and 9D, the coordinate measuring machine can appropriately correct the displacement of the probe and the measurement error can be reduced.
On the other hand, when the measurement conditions during the probe calibration and the profiling measurement are different, as shown in FIGS. 8B and 8C and FIGS. 9B and 9C, the coordinate measuring machine cannot appropriately correct the displacement of the probe and the measurement error is enlarged. Incidentally, though the push amount and the measurement direction are varied in FIGS. 8A to 8D and FIGS. 9A to 9D, the same applies when the measurement speed is varied.